The current iMac model has two memory slots. That is all that the memory
controller will support, so Apple didn't have the option of adding more

Apple didn't have the option of using a different memory controller?

Time to market, resource availability at the time, no burning need to do
something that the Intel support chipset would do 95% of for far lower
total cost to bring to market, ...

So they did have the option and for business reasons declined.

Well within their rights, and an understandable decision.

But not "didn't have the option."

Given time constraints and cost/resource issues, "didn't have the
option" can often come very near fact.

Having the luxury of lots of available options isn't always something
any company can enjoy; "for business reasons declined" is often pretty
much indistinguishable from not having the option at all.

Of course, this is from someone looking at things from inside
engineering development for the past 30 years. Very often time available
prunes your decision tree for you. And that's not always a bad thing.
.